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ABC of AAC Autoclaving

The primary purpose of the Autoclaving is to transform the green cake into white cake.
This transformation takes place in autoclaves, where the green cakes are left for curing.
The actual curing takes place by filling the autoclave with pressurised steam (saturated water vapour) supplied by the boiler.
It is during this environment where the most reaction will happen between the sand or PFA and therefore the binding agents lime and cement.
This reaction, which results in the forming of calcium silicate hydrates within the cellular concrete’s matrix, takes place in two reaction phases.
In the first phase, a topochemical reaction takes place during which the lime and silica acid are the reactants.
Water doesn't happen within the actual reaction; it’s only built into the new formulations that are formed.
These reactions occur within the border areas of the raw materials, starting at the active spots (e.g. on the edges) of a crystal.
The second phase of the reaction will mainly involve the solvent phase where the lime and silica acid, both of them are dissolved in water, will react into insoluble calcium silicate hydrates.
These hydrates will form a structure.
Before steam is often entered into the autoclave, the autoclave must be evacuated by reducing the pressure within the autoclave with the utilization of vacuum from the air pump or steam ejector.
Hereby the existent atmosphere within the autoclave is removed.
By keeping the evacuation pressure below the saturation pressure of water, which is out there within the green cakes, steam is released from the green cakes as a desired side effect.
Air and hydrogen which are still present within the green cakes are scavenged from the autoclave by this self-formed saturated steam.
At a pressure of 0.3 bar (70% vacuum) the saturation temperature of water is about 69°C.
The heat transfer during the heat-up phase of the curing cycle is therefore reduced and can also end in a reduced curing time.
After the autoclave has been evacuated and heated the autoclave is kept at a curing pressure of about 12 to 13 bar and a temperature of about 198°C and is completed with the blowing-off phase of the curing process after which the autoclave are often opened again to unload the white cakes.
After the steam curing maximal compression strength is obtained with a minimal amount of shrinkage.
Any sudden disruption in pressure and or temperature during any phase of the curing process will end in damaged AAC products.
The autoclaves are long pressure vessels which will withstand the pressure and temperature changes that the autoclaves are subjected to during the curing process.
The autoclaves are mounted under a small slope to facilitate the draining of condensate that's formed during the curing process.
In the autoclaves rails are mounted to be ready to load and unload trolleys or wagons or bogeys into/from the autoclaves.
The autoclaves are opened and closed with a door that's operated manually or pneumatically.
For safety reasons, switches are installed to make sure whether an autoclave is open or closed and locked.
The autoclaves are covered with a thick layer of insulation for safety reasons, to guard humans from excessive heat, and to enhance the efficiency of the curing process.
The latter improving the economic feasibility and to scale back the impost on the environment.
As stated before, condensate are going to be formed during the curing process within the autoclaves.
This condensate must be drained from the autoclaves during a safe and controlled manner.
The condensate is therefore led through the recent condensate pressure reducer, in order that this condensate, the so called dirty condensate, is often safely fed into the condensate pit.
The level control equipment, dirt traps, etc., to be mounted within the overflow condensate pit, are a part of the module overflow pit condensate accessories.
The dirty condensate is converted into clean condensate and is mixed with water from the Utilities within the overflow condensate pit where insoluble particles are far away from the condensate.
The cleaned condensate is transported as condensate to the slurry preparation section to be reused within the production of AAC.
As has been described before there's variety of pressure vessels involved within the Autoclaving.
These pressure vessels are constructed, tested, certified and installed according Indian legislation. Once the plant is operational however, it's the purchaser’s responsibility to take care of these pressure vessels and therefore the steam preparation plant, and have them periodically re-certified consistent with the purchaser’s local legislation and regulations.


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